I've got it working pretty reliably now. Still lots of cosmetic stuff to do.
The basic movement is done by 2 control rods, one on a crank and the other (bow arm) on a cam. I've gone through about 9 iterations of the cam, and I've just made a new one, to try and get the two arms coming up together. It works very well and is a big improvement.
In the video you can see the bow arm comes up a bit too early ( or a medieval style draw).
There has been an immense amount of fiddling and fettling, but it all slowly improves the reliability and repeatability.
It's motorised now so that pressing a button starts it, and then it stops after the arrow is loosed, as the microswitch is actuated (see bottom pic)
https://www.youtube.com/watch?v=eDJvVBdDx54
